In a motor vehicle with a three-phase electric motor, the motor is controlled in a known manner by using three AC current-mode control signals delivered by a control device.
In one existing solution illustrated in FIG. 1, this device 1A comprises a microcontroller 10, an amplifier 20 and a power stage taking the form of an inverter 30 allowing these three signals U, V, W for current-mode controlling the motor 2 to be generated.
More specifically, the microcontroller 10 first generates six low-amplitude pulse-width modulated (PWM) AC primary signals (two per phase of the motor 2), the amplitude of which corresponds to the amplitude of the power supply signal of the microcontroller 10, for example of the order of 5 V. These signals each alternate between high states (or active states) and low states (or inactive states).
These signals are next amplified by the amplifier 20 so as to reach for example an amplitude of the order of 20 V (when the battery voltage B of the vehicle is of the order of 10 V, for example) then transmitted to the inverter 30 which delivers, as output, the three signals U, V, W for current-mode controlling the motor 2, which also exhibit alternation between high states and low states. Each phase of the motor 2 is then controlled in a known manner by the offset existing between two of the three control signals U, V, W, pairwise.
For this purpose, the inverter 30 comprises three pairs of MOSFET transistors, each generating one of the three signals U, V, W for current-mode controlling the motor 2. One of the transistors (the high-stage transistor) in each transistor pair is connected to the positive terminal of the battery B of the vehicle while the other (the low-stage transistor) is connected to ground M. The two transistors of a pair each receive an amplified primary signal, these two amplified primary signals being centered (i.e. the centers of the pulses coincide) but inverted. In practice, the pulses of these amplified primary signals are not square but exhibit a rising slope and a falling slope.
To prevent the two transistors of one and the same pair allowing the current through at the same time, which would connect the battery voltage to ground (short-circuit) and could damage the control device 1A or even the motor 2, the microcontroller 10 observes a dead time between a pulse of the primary single intended for the high stage of a given pair of transistors and the pulse associated with the primary single that is intended for the low stage of said pair. In practice, as illustrated in the example given in FIG. 2, the two high-stage SEH1 and low-stage SEB1 signals remain centered but the width of one of the two primary signals (the high-stage signal SEH1 in this example) is decreased so as to prevent the two signals SEH1 and SEB1 being in the active state at the same time.
Examples of signals of the control device 1A (illustrated for a PWM signal cycle) are shown in FIGS. 3 to 6. More specifically, FIG. 3 illustrates a high-stage signal SEH1 generated by the microcontroller 10 intended for a high-stage transistor of a pair of transistors, FIG. 4 illustrates the output signal SEH2 of said high-stage transistor, FIG. 5 illustrates a low-stage signal SEB1 generated by the microcontroller 10 intended for the low-stage transistor of said pair of transistors and FIG. 6 illustrates the output signal SEB2 of said low-stage transistor. It can thus be seen in FIGS. 3 and 5 that the microcontroller 10 observes a dead time MotPwmDiffTheo between the end t1 of an active state of the low-stage signal SEB1 and the start t2 of an active state of the high-stage signal SEH1. It can be seen in FIG. 4 that an initiation delay d1 followed by a rise time d2 is required by the high-stage transistor for the output signal SEH2 of the transistor to reach the active state, then a deactivation delay d3 for the transistor followed by a fall time d4 for the output signal SEH2 is required for the transistor to switch to the inactive state. Similarly, it can be seen in FIG. 6 that a deactivation delay d5 followed by a fall time d6 is required by the low-stage transistor for the output signal SEB2 of the transistor to reach the inactive state, then an initiation delay d7 for the transistor followed by a rise time d8 for the output signal SEB2 is required for the transistor to switch to the active state.
The dead time MotPwmDiffTheo observed by the microcontroller 10 between two primary signals intended for a pair of transistors, referred to hereinafter as the inserted dead time, is limited to 500 ns so as not to negatively affect the performance of the motor 2. Specifically, an increase in the duration of the dead time MotPwmDiffTheo results in a decrease in the width of the high states SEH1 of the signals U, V, W for controlling the motor 2 and hence in less effective current-mode control of the motor 2, in particular beyond 500 ns of dead time.
To diagnose a fault in the control of the motor 2, it is known practice to measure the actual dead time present in the control signals. In a first existing solution, the six primary signals SEH1, SEB1 generated by the microcontroller 10 are measured. Such a solution however makes it possible only to diagnose a fault in the dead times inserted by the microcontroller 10, i.e. a fault in the microcontroller 10 itself. However, it is observed that the duration of the dead time inserted by the microcontroller 10 into the primary signals SEH1, SEB1 may be modified by the amplifier 20 or the inverter 30. Specifically, when using the vehicle, the transistors of the inverter 30 may heat up and modify the duration of the dead times of the control signals U, V, W output by the inverter 30, which may negatively affect the current-mode control of the motor 2. Therefore, in a second existing solution, the device 1A comprises a measurement module 40 which determines the rise times of the control signals U, V, W and compares them with the rise times of the primary signals SEH1, SEB1 generated by the microcontroller 10. However, such a solution does not make it possible to detect a fault in the microcontroller 10. Moreover, the latency time taken for such calculations allows an accuracy only of the order of a microsecond to be obtained, i.e. twice the maximum duration of a dead time (500 ns), particularly as the dead time is affected by the effects of the current flowing through the transistors, the measurement errors caused by the measurement module 40, the delays introduced by the electronic filters, and the rise and fall times of the primary signals SEH1, SEB1 and of the control signals U, V, W, thereby making the method inaccurate or even random and therefore presenting a major drawback.